1. Field of the Invention
The present invention relates to a latent curing agent for epoxy curing resins. More particularly, it relates to a latent curing agent for epoxy resins which causes rapid resin curing at moderate, elevated temperatures and which gives epoxy resin compositions having excellent storage stability at room temperature.
2. Description of the Prior Art
Epoxy resins are widely employed as electric insulating materials, various molded products, adhesives or coatings, because they give valuable cured resins having excellent mechanical, electrical and chemical properties when cured with suitable curing agents for example acid anhydride and amine curing agents. However, epoxy resin composition incorporating amine curing agents cure rapidly at ordinary temperatures and at elevated temperatures and hence they lack storage stability. Also, epoxy resin compositions which incorporate acid anhydride curing agents are stable at ordinary temperatures, but heating for a long period of time at elevated temperature is required for full curing. Usually, tertiary amines, quaternary ammonium compounds or organometal complexes are further added to the composition for purposes of accelerating curing rate. However, the addition of such cure accelerators impairs storage stability markedly.
There have been eagerly desired so-called latent curing agents which are compatible with epoxy resins to form composition which is stable at relatively low temperature and which is rapidly cured when heated to elevated temperature. Representative compounds which have been heretofore proposed as latent curing agents are dicyandiamide, dibasic acid hydrazide, boron trifluorideamine adduct, guanamine, and melamine. Among these compounds, dicyandiamide, dibasic acid hydrazide and guanamine are useful in formulating epoxy resin compositions which have excellent storage stability but full curing by means of these compounds could be achieved by heating at higher temperature than 150.degree. C. for a long time. Also, boron trifluoride-amine adduct is hard to treat owing to its highly hygroscopic properties and it affects adversely upon the physical properties of the cured resin.
There has been heretofore known almost no latent epoxy curing agent which causes rapid curing at moderately elevated temperature, that is 100.degree. C.-150.degree. C. and which gives epoxy resin composition having excellent storage stability at ordinary temperature. The epoxy resin compositions which comprise the latent epoxy curing agent are so-called one pack-type epoxy resins.
One-pack type epoxy resins are preferable to the conventional two-pack type epoxy resins because the former cannot be misformulated and can be used continuously. A need therefore continues to exist for an improved curing agent for a one-pack type epoxy resin.